what is the equation of a stream line passing through the point (3, 1)?

Question

         If u = ax and  y = - ay give the velocity distribution for a two dimensional flow what is the equation of a stream line passing through the point (3, 1)?

         (a) xy = 3            (b) x + y = 4    (c) x + 3y = 6              (d) x²y = 9